"The plan is to destabilise the situation, the plan is for foreign troops to cross the border and seize the country's territory, which we will not allow,'' he said, adding that people engaged in the unrest have distinct Russian accents.

He said Russian troops remain stationed within 30 kilometres (19 miles) of the frontier.

The city of Luhansk is just 25 kilometres (15 miles) west of Russia.

Reinforcements

Police have blocked roads into Luhansk and armed reinforcements are being sent to the restive cities.

Officials said Ukrainian National Security and Defence Council Secretary Andriy Parubiy and Security Service chief Valentyn Nalyvaychenko have been sent to the city.

Interior Minister Arsen Avakov had already arrived in Kharkiv, and First Deputy Prime Prime Minister Vitaly Yarema is on his way to Donetsk, a spokeswoman said.

She said they had "all the authority necessary to take action against separatism."

Ukraine's interim President Olexander Turchynov cancelled a visit to Lithuania to deal with the unfolding events.

The crisis has heightened nervousness in many other eastern European states, with Czech President Milos Zeman saying NATO should deploy troops in Ukraine if Russia invades.

"If Russia decides to extend its territorial expansion to eastern Ukraine, the fun is over," he told Czech public radio on Sunday.

Tensions have escalated in eastern Ukraine in recent weeks.

Russia is consolidating its grip on Crimea, annexed by Moscow last month, and thousands of Russian troops remain massed near the Ukrainian border.

The latest developments come as Ukraine's defence ministry said a Russian soldier had killed a Ukrainian military officer still loyal to Kiev in eastern Crimea late on Sunday.

Another Ukrainian officer present is reported to have been beaten and detained by Russian troops in Sunday's incident in the small town of Novofyodorovka.

The circumstances of the incident are unclear.

A Ukrainian defence ministry spokesman said the soldier had been preparing his belongings to leave Crimea in a few days, when an argument broke out with Russian servicemen.

He was then shot twice on the fifth floor of the dormitory where he lived.

But Russian media reports said a group of Ukrainian soldiers had been drinking and were on their way home when they passed Russian soldiers guarding an entry to the military base where they previously worked, prompting an argument between the two groups.

Russian news agencies reported that prosecutors have opened a criminal investigation into the death.

'Destructive' NGOs

On Monday, President Vladimir Putin told the FSB security service to be on the alert against militant attacks and said Russia must not let non-governmental organisations be used for "destructive" purposes "as in Ukraine".

Also on Monday, Russia's consumer protection agency said it had suspended imports from six Ukrainian dairy producers, after finding their products violated regulations during laboratory tests.